1. Field of the Invention
The present invention relates to a wavelength multiplexing apparatus for multiplexing and/or demultiplexing a plurality of optical signals in the wavelength region and a signal conversion apparatus for interfacing a transmission line which corresponds to any of these optical signals with the wavelength multiplexing apparatus.
2. Description of the Related Art
Recently, technology for realizing optical devices such as a light source with stable wavelength and level, an optical combiner with narrow band has been progressed significantly. For this reason, wavelength division multiplexing (WDM) has been actively applied to the trunk line system and the access network.
FIG. 34 is a diagram showing a first structural example of a node adapted to the wavelength division multiplexing.
In the drawing, a common external clock signal is supplied to OEO converting parts 110-1 to 110-N. Up links of full-duplex optical transmission lines 111-1 to 111-N are connected to corresponding multiplexing inputs of a wavelength multiplexing section 112 through the OEO converting parts 110-1 to 110-N, respectively. The wavelength multiplexing section 112 includes a plurality N of demultiplexing outputs, which are connected to down links of the optical transmission lines 111-1 to 111-N through the OEO converting parts 110-1 to 110-N, respectively. A multiplexing output and a demultiplexing input of the wavelength multiplexing part 112 are connected to one end of a full-duplex optical multiplex transmission line 113 which is adapted to the wavelength division multiplexing.
The OEO converting part 110-1 is composed of the following elements:    an OE converting part 124u-1 and a synchronization part 125u-1 which are cascaded to the up link of the optical transmission line 111-1,    a wavelength converting part 126u-1 which is arranged on a subsequent stage of the synchronization part 125u-1, and whose output is connected to the corresponding multiplexing input of the wavelength multiplexing part 112,    an OE converting part 124d-1 and a synchronization part 125d-1 which are cascaded to the corresponding demultiplexing output of the wavelength multiplexing part 112,    a wavelength converting part 126d-1 which is arranged on a subsequent stage of the synchronization part 125d-1, and whose output is connected to the down link of the optical transmission line 111-1,    a clock receiving part 127-1 whose input is supplied with the aforementioned external clock signal,    a clock generating part 128-1 for generating an internal clock signal which can substitute the external clock signal,    clock extracting parts 129u-1, 129d-1 which are connected to monitor terminals of the OE converting parts 124u-1, 124d-1, and    a clock selecting part 130-1 which is connected to outputs of the aforementioned clock receiving part 127-1, the clock generating part 128-1 and the clock extracting parts 129u-1, 129d-1, and whose output is connected to clock inputs of the synchronization parts 125u-1, 125d-1.
Incidentally, the structures of the OEO converting parts 110-2 to 110-N are the same as that of the OEO converting part 110-1 and hence, in the following explanation, the same reference numerals, added with subscripts “2” to “N” instead of “1”, are given to designate the elements with the same function and structure and explanations thereof are omitted.
The wavelength multiplexing part 112 is composed of the following elements:    an optical coupler 131u which is connected to outputs of the wavelength converting parts 126u-1 to 126u-N, provided in the OEO converting parts 110-1 to 110-N, via optical fibers, respectively,    a transmitting amplifier 132u which is arranged on a subsequent stage of the optical coupler 131u and whose output is connected to an up link of the optical multiplex transmission line 113,    a receiving amplifier 132d which is connected to a down link of the optical multiplex transmission line 113, and    an optical filter 131d which is arranged on a subsequent stage of the receiving amplifier 132d and whose outputs are respectively connected to inputs of the OE converting parts 124d-1 to 124d-N provided in the OEO converting parts 110-1 to 110-N.
Note that, in the following explanation, the elements which are common in the OEO converting parts 110-1 to 110-N are designated by using the subscript “c”, meaning that it can correspond to any of the subscripts “1” to “N”.
In the OEO converting part 110-c, the OE converting part 124u-c converts an optical signal, which is supplied via the up link of the optical transmission line 111-c and modulated according to transmission information, to an electrical signal (hereinafter referred to as an “upstream signal”).
Moreover, a plurality N of optical signals (hereinafter referred to as “downstream optical signals”), each of which corresponds to the OEO converting parts 110-1 to 110-N (optical transmission lines 110-1 to 110-N), respectively, and whose wavelength is different from each other, are multiplexed to generate a wavelength-multiplex signal (hereinafter referred to as a “downstream optical multiplex signal”), which is supplied to the receiving amplifier 132d provided in the wavelength multiplexing part 112 via the down link of the optical transmission line 113.
The optical filter 131d demultiplexes the downstream optical multiplex signal, which is supplied through the receiving amplifier 132d, in the wavelength region, thereby supplying the corresponding downstream optical signals to the OEO converting parts 110-1 to 110-N, respectively.
In the OEO converting part 110-c, the OE converting part 124d-c converts thus-distributed downstream optical signal to an electrical signal (hereinafter referred to as a “downstream signal”).
The clock extracting parts 129u–c, 129d–c generate an upstream clock signal and a downstream clock signal, each of which is synchronized to the aforementioned upstream signal and the downstream signal.
Further, the clock receiving part 127-c subjects the aforementioned external clock signal to predetermined processing (such as waveform shaping).
Furthermore, the clock generating part 128-c regularly generates the predetermined internal clock signal.
The clock selecting part 130-c selects any clock signal (it is supposed as the “external clock signal” for simplifying the explanation and referred to as a “reference clock signal” in the following explanation) out of the aforementioned upstream clock signal, the downstream clock signal, the external clock signal and the internal clock signal, which corresponds to a maintenance and operation system, as well as office data. Incidentally, in the following explanation, it is supposed that the frequency of the reference clock signal is 1.544 MHz for simplifying the explanation.
The synchronization part 125u–c applies re-timing to the upstream signal which is supplied from the OE converting part 124u–c, in synchronization with the reference clock signal, thereby generating a “wave-shaped upstream signal”.
The wavelength converting part 126u–c generates an optical signal with a predetermined wavelength modulated by the wave-shaped upstream signal (hereinafter referred to as an “upstream optical signal”).
In the wavelength multiplexing part 112, the optical coupler 131u multiplexes a plurality N of the upstream optical signals, which are generated in parallel by the OEO converting parts 110-1 to 110-N like the above, thereby generating a wavelength-multiplex signal (hereinafter referred to as an “upstream optical multiplex signal”).
The transmitting amplifier 132u transmits the upstream optical multiplex signal to the up link of the optical multiplex transmission line 113 with a predetermined level.
Further, in the OEO converting part 110-c, the synchronization part 125d–c applies re-timing to the downstream signal which is supplied from the OE converting part 124d–c, in synchronization with the reference clock signal, thereby generating a “wave-shaped downstream signal”.
The wavelength converting part 126d–c generates an optical signal with a predetermined wavelength modulated according to the wave-shaped downstream signal and sends out the optical signal to the down link of the optical transmission line 111-c. 
Therefore, the optical transmission lines 111-1 to 111-N and the optical multiplex transmission line 113 are synchronized with each other while flexibly corresponding to the aforementioned office data and the maintenance and operation system, and, under the synchronization, predetermined transmission information is transferred accurately and stably based on the wavelength division multiplexing.
Incidentally, the above-described conventional example is referred to as a “first conventional example” in order to discriminate it from a later-described conventional example.
FIG. 35 is a diagram showing a second structural example of a node adapted to the wavelength division multiplexing.
Differences of the structure of the drawing from that of the node shown in FIG. 34 are that a single or a plurality n (≦N) of supervisory and control part(s) 200-1 to 200-n, each of which has a port connected to a control terminal provided respectively to a predetermined number of OEO converting parts out of the OEO converting parts 110-1 to 110-N, is/are provided thereto, and that an MC part 201 which has communication ports connected respectively to communication ports of the supervisory and control parts 200-1 to 200-N and the wavelength multiplexing part 112.
It should be noted that the basic structures of the OEO converting parts 110-1 to 110-N and the wavelength multiplexing part 112 are the same as those shown in FIG. 34, and hence illustrations thereof are omitted for simplifying the explanation.
In thus-structured node (hereinafter referred to as a “second conventional example”), the supervisory and control parts 200-1 to 200-n transfer a command (showing an operation mode of the respective OEO converting parts) and a status (operation condition) in parallel, which should be delivered between the subordinate OEO converting part, out of the OEO converting parts 110-1 to 110-N, and the MC part 201.
The MC part 201 takes the initiative to perform predetermined processing of the maintenance and the operation of the aforementioned OEO converting parts 110-1 to 110-N and the wavelength multiplexing part 112, and, based on its processing procedure, exchanges the command and the status reciprocally between the OEO converting parts 110-1 to 110-N and the wavelength multiplexing part 112, through the communication ports and the supervisory and control parts 200-1 to 200-n. 
Therefore, a load (throughput) which is necessary for delivering the command and the status reciprocally between the OEO converting parts 110-1 to 110-N and the MC part 201 is dispersed by the supervisory and control parts 200-1 to 200-n. 
Namely, the throughput of the MC part 201 and the number of the communication ports to be provided to the MC part 201 are kept at small values, even when the number N of the OEO converting parts 110-1 to 110-N to be equipped thereto differs substantially for each node or expands substantially during the operation.
Therefore, the structure standardization and the cost reduction can be achieved with high reliability as well as flexible adaptation to various forms of maintenance and operation, and maintaining of high total reliability.
It should be mentioned that, in the above-described first conventional example, the external clock signal, which should be supplied to all of the OEO converting parts 110-1 to 110-N, has to be supplied via lines such as coaxial cables which are in advance connected to the corresponding terminals of the OEO converting parts 110-1 to 110-N. However, since the external clock signal is selected according to the office data and the forms of the maintenance and operation, it is not necessarily shared.
Moreover, a source of the external clock signal like the above and the OEO converting parts 110-1 to 110-N are not necessarily located in the same office premises or on the same floor.
For this reason, it is difficult to realize synchronization with the individual external clock signals which correspond to a combination (group) of the optical transmission lines having a predetermined property in common (for example, an operator such as a telecommunication carrier and a user), due to its cost increases and various limitations caused by construction and the like.
Moreover, complicated wiring work (including wire binding) is necessary in order to underlay the lines for supplying the external clock signal to the OEO converting parts 110-1 to 110-N. Hence, it is demanded, not only to reduce the cost, but also to flexibly correspond to the various arrangements of the OEO converting parts 110-1 to 110-N.
Further, the number of the optical signals to be multiplexed to the wavelength-multiplex signal (hereinafter referred to as the “multiplicity”) has been rapidly increased as the technology progresses, and therefore, the length of the line is increasing.
Therefore, there are high possibilities that the level of electromagnetic interference (EMI) which is radiated from the line is increased, and that a substantial deviation is caused in a waveform of the external clock signal which is supplied respectively to the OEO converting parts 110-1 to 110-N.
Furthermore, in the above-described second conventional example, communication links which are formed between the OEO converting parts 110-1 to 110-N and the nodes, and the supervisory and control parts 200-1 to 200-n (hereinafter referred to as “specific communication links”), communication links which are formed between the supervisory and control parts 200-1 to 200-n and the MC part 201 (hereinafter referred to as “combined communication links”) and a communication link which is formed between the wavelength multiplexing part 112 and the MC part 201 (hereinafter referred to as a “shared communication link”) are formed as the electrical full-duplex transmission lines.
Therefore, the level of the electromagnetic interference (EMI) which is radiated from the communication lines like the above is increased as the number of the communication links increases, and causes the external clock signal to deteriorate its waveform. Also, there is a possibility that it acts as disturbance on the external apparatus and the transmission system.
Further, the aforementioned specific communication links and the combined communication links are substantially larger in number than the shared communication link, although the OEO converting parts 110-1 to 110-N may expand, and formed in a mesh-state between the OEO converting parts 110-1 to 110-N and the supervisory and control parts 200-1 to 200-n, and the supervisory control parts 200-1 to 200-n and the MC part 201.
Therefore, the complicated wiring work (including the wire binding) is necessary in order to underlay the specific communication links and the combined communication links, and hence, it is demanded, not only to reduce the cost, but also to flexibly correspond to the various arrangements of the OEO converting parts 110-1 to 110-N, similarly to the aforementioned underlaying of the lines for supplying the external clock signal to the OEO converting parts 110-1 to 110-N.